The present invention is in the field of glass-ceramics and particularly relates to the manufacture of light-absorbing glass-ceramic products which are blue-to-black in color.
Black glasses are well known and are ordinarily produced by the use of coloring agents such as cobalt, nickel, manganese, chromium, iron and the like. The use of titanium as a colorant in glass is noted in U.S. Pat. No. 1,919,264, which teaches that black borosilicate glasses can be produced by adding iron, titanium and, preferably, a reducing agent such as starch to the glass batch.
Glass-ceramic materials are crystalline or semicrystalline materials produced by the in situ crystallization of glasses. Crystallization is accomplished in accordance with a process comprising, first, compounding and melting a batch for a glass which includes nucleating agents to promote glass crystallization; secondly, forming the melt into a glass article of the desired configuration; and thirdly, heat treating the glass article in accordance with a time-temperature schedule which promotes the formation of crystal nuclei and the subsequent growth of crystals thereon as the treatment proceeds. This process provides a product composed of highly crystalline material wherein the crystals are uniform in size, fine-grained, and evenly distributed throughout a minor residual glassy matrix.
The proportion of crystals in most glass-ceramic materials is quite high, being over 50% and usually over 75% by weight. Hence glass-ceramic materials typically exhibit properties more closely related to those of the crystal phase than to those of the parent glass. A good general discussion of the principles underlying the manufacture of glass-ceramics is found in U.S. Pat. No. 2,920,971 to Stookey, and reference may be made to this patent and to numerous other publications for further information relating to this field.
The in situ crystallization of glasses in the Li.sub.2 O-A1.sub.2 O.sub.3 -SiO.sub.2 composition field in accordance with the above-described procedures can provide highly crystalline glass-ceramics wherein beta-quartz and/or betaspodumene constitutes the principal crystal phase. Such glass-ceramics have properties making them especially useful for high temperature and temperature cycling applications. The relatively high strength and low linear coefficient of thermal expansion of these materials make them prticularly resistant to breakage by thermal shock.
Beta-quartz and beta-spodumene glass-ceramics containing TiO.sub.2 as a nucleating agent have been reported in a number of issued patents, including U.S. Pat. Nos. 2,960,801; 3,148,994; 3,157,522; 3,380,818; 3,625,718; 4,018,612 and many others. The appearance of such glass-ceramics can be varied by varying composition and heat treatment; hence transparent, translucent or opaque glass-ceramics which are water-white, opaque white or variously colored have been produced and described in the prior art.
A black glass-ceramic material capable of absorbing visible and near infrared radiation would have utility for a variety of technical and consumer-related applications, including, for example, stove-tops, cookware, open viewports, dark protective lenses and the like. Transparent black materials would be useful for viewport applications, whereas cookware could be fabricated from black opaque materials.
It is the principal object of the present invention to provide a convenient and efficient method for producing blue-to-black beta-spodumene and/or beta-quartz glass-ceramic materials without requiring the use of added oxide coloring constituents having possible adverse effects on glass-ceramic properties.
It is a further object of the invention to provide black glass-ceramics containing beta-spodumene and/or beta-quartz as the principal crystal phase, produced by the method hereinafter set forth.
Other objects and advantages of the invention will become apparent from the following description thereof.